I expect both teams to hold onto their players.
Thompson is a combo big who can play both big man positions and can complement both Carl Landry (disappointing since joining Sacramento) + DeMarcus Cousins. Meanwhile, Anthony Randolph is still one of the Warriors top two players/prospects along with Steph Curry. They also feel that Randolph can play the 3, 4 and 5 and that the 5 might be his best position long term (Nellie's words).
It would take a major trade package, good young talent, to pry either of these players away from either of those teams.
Between Cousins, Dalembert, and Landry, you think there will be more than 10-15 minutes left each game for Jason Thompson? Not to mention Omri Casspi, who often gets minutes at the 4?
On the Warriors, between Randolph, Wright, Udoh, and Biedrins, I imagine one of those has to be an odd man out as well. Randolph is far too thin to play C but doesn't have the shooting ability to play SF. The Warriors also have Ronny Turiaf still on contract, I believe.
I would tend to agree with you that the teams would keep their players if it weren't for the fact that the Kings are bleeding money and the Warriors are desperate to shed salary in order to make the team attractive to buyers.
Surely prying one of their excess young bigs off of them in order to help them save money and free up more minutes for their new projects is not all that unreasonable. We may have to take on an additional contract while doing so, but as long as it is not too burdensome (e.g. Monta Ellis), it would be a really good idea.
The Celtics desperately need a young player with potential, size, and athleticism. I think trading with the Kings or the Warriors is the best opportunity for acquiring such a player given what the Celtics have to trade.